Titanium dioxide pigments are used in many applications. One particular application demanding light fastness is the use in paper incorporated into paper laminates for decorative applications.
In this application the titanium dioxide pigmented paper is saturated with a laminating resin and subjected to heat and pressure to produce a hard surface laminate in which oxygen is absent. The titanium dioxide on exposure to UV light tends to gray as the concentration of Ti3+ ions are produced without the continual conversion by air oxidation of the ions back to the Ti 4+ state.
Many techniques have been employed to make a light fast titanium dioxide pigment from the use of a calcination step in the pigment manufacture to the use of redox couples such as Ce2+/Ce4+ to keep the titanium ion in the colorless 4+ oxidation state. But the use of such oxidation couples may lead to a yellowing of the titanium dioxide. Calcination on the other band adds time and cost to pigment manufacture.
In the present invention the objective was to find a noncolor producing method by which titanium dioxide pigment could be light stabilized. This was achieved by a process of precipitating a single layer coating of alumina phosphate on the surface of a titanium dioxide starting pigment. The product produced by the present process proved to be water dispersible and to exhibit a high retention in the paper making process.
Prior art methods of making a light fast titanium dioxide include U.S. Pat. Nos. 5,976,237; 5,785,748; 5,665,466; 4,400,365; and 3,853,575.
U.S. Pat. No. 5,976,237 to Halko et al. teaches a process for making durable pigments for plastics that may include a treatment step including a compound which is a source of P2O5. The Halko process requires that the pigment surface is first treated with alumina and silica and optionally another oxide such as a source of P2O5.
U.S. Pat. No. 5,785,748 to Banford et al. teaches the use of a prepared reagent that is a mixture of an aluminum compound and phosphoric acid prepared under heating. The source of the aluminum compound must be one that will dissolve in phosphoric acid. The aluminum compound is dissolved and the solution is clear, the mixture is diluted and added to a slurry of the titanium dioxide starting pigment. The pH is then adjusted to about 3.5 to 5 to precipitate the treatment on the pigment surface. For improved light fastness, a compound believed to be an oxidizing agent, such as potassium iodate, copper sulfate or potassium nitrate, is added in the process.
U.S. Pat. No. 5,665,466 to Guez et al. teaches a process for making a titanium dioxide of at least two layers. The first layer is an aluminum phosphate layer and the second is an aluminum oxide layer. The pigment is characterized by a positive zeta potential at high pH to ensure high physicochemical retention of the pigment in the paper.
U.S. Pat. No. 4,400,365 to Haacke et al. teaches a combination of aluminum and zinc phosphates as a means to increase titanium dioxide lightfastness, and U.S. Pat. No. 4,052,224 teaches a treatment of using compounds of phosphorus, aluminum, zirconium, titanium and silica to increase light fastness.